The information recording medium, namely, optical disc or compact disc (CD), on which information can be recorded by means of a laser beam has been recently developed. At present, the information recording medium is utilized, for instance, as a video disc, an audio disc, a large capacity filing means for storing a still picture, a large capacity computer data disc. Among the known compact discs, a recordable compact disc (CD-R), which is also named a compact disc of Direct-Read-After-Write (DRAW) type is paid much attentions because it is expected to be appropriately employable for computer data storage. The CD-R has an advantage in that computer data can be easily stored using a CD-R recording device to produce a data recorded medium.
The optical disc of CD-R type generally comprises a disc-shaped substrate and a recording layer provided thereon. The disc substrate comprises glass or synthetic resin. The recording layer comprises a metal or a semi-metal such as Bi, Sn, In or Te, or alternatively a dye such as a cyanine dye, a metal-complex dye, or a quinone dye. An intermediate layer of polymer material may be arranged between the substrate and the recording layer, whereby increasing smoothness of the surface of substrate, increasing adhesion between the substrate and the recording layer, or increasing sensitivity of the recording layer.
On the CD-R, the writing (recording) and reading (reproducing) can be performed in the following manner. The laser beam modulated by information is applied to the recording layer of the CD-R through the substrate. The recording layer alters its optical characteristics in the area where the laser beam has been applied, to produce a chemical or physical change such as production of pit. The reading of the recorded information can be performed by sequentially applying a laser beam on the recording layer of the CD-R through the substrate and detecting a light reflected on the CD-R.
At present, most of the recording layer of CD-R utilizes a light-sensitive dye as the recording material, because the recording dye layer can be easily formed by a coating method and shows a high sensitivity, as compared with the recording metal layer. However, the dye layer has such disadvantages as low light refection, low C/N of reproduction signal, low light resistance, and poor durability.
Japanese Patent Provisional Publication No. 64-40382 describes that an optical disc having a recording layer of a cyanine dye which has a benzoindolenine skeleton shows high light reflection and enhanced C/N.
Japanese Patent Provisional Publication No. 64-40387 describes an optical disc having a recording layer of a combination of a dicarbocyanine dye (having 5 methine units) having a benzoindolenine skeleton (i.e., structure in which a benzene ring is fused with the indolenine skeleton) and a tricarbocyanine dye (having 7 methine units) having an indolenine skeleton. The recording layer of this type utilizes a high light reflection of the dicarbocyanine dye having benzoindolenine skeleton and a high C/N of the tricarbocyanine dye having the indolenine skeleton.
Nikkei Electronics (in the Japanese language, page 107, issued on Jan. 23, 1989) describes a provision of a light-reflecting layer on the recording dye layer. This publication further describes that the dye of the recording layer melts in the laser beam-applied area and the plastic substrate on the area protrudes into the melted recording layer, whereby a pit is produced on that area.
Although a number of studies have been made on the recording layer of light-sensitive dye type, light resistances of the known dyes are still not satisfactory. For this reason, a singlet oxygen quencher is generally combined with the light-sensitive dye.
Japanese Patent Provisional Publication H2-300288 describes an optical disc having a recording layer which comprises a cyanine dye and a nitroso compound and is stated to be enhanced its light-resistance. Japanese Patent Provisional Publication H4-146189 describes an optical disc having a recording layer which comprises a combination of a dicarbocyanine dye (containing five methine units) and a nickel complex compound and is stated to show enhanced light resistance. These combinations utilize the known singlet oxygen quenchers such as the nitroso compound and the nickel complex compound, whereby the light-resistance of the cyanine dye is increased. The use of the nitroso compound, however, has a disadvantageous feature. In more detail, if the nitroso compound should be used in a great amount if the light-resistance is desired to effectively increase. However, the use of a great amount of the nitroso compound unfavorably decreases light reflection. The nickel complex compound, namely, a heavy metal-containing compound, may cause environmental pollution.
Japanese Patent Provisional Publication No. 63-64794 proposes a light information recording medium having a recording layer which comprises a cyanine dye and an electron-acceptable compound and is stated to show improved light-resistance. An example of the cyanine dye is a tricarbocyanine dye (containing seven methine units) having a benzoindolenine skeleton. Examples of the electron-acceptable compounds are tetracyanoqunodimethane (TCNQ) and tetracyanoethylene. The combination of the disclosed cyanine dye and electron-acceptable compound, however, has been found to be still not satisfactory in the light-resistance. Particularly, the light-resistance after the information is recorded, that is, archival light-resistance, is un satisfactory.
Accordingly, it is an object of the present invention to provide a light information recording medium showing high light-resistance and high durability, as well as high recording and reproducing characteristics.
Particularly, it is an object of the invention to provide a light information recording medium of CR-R type which shows sufficient light-resistance and durability even after the information is recorded.